The present invention relates to a system for automatically detecting the presence of moisture on a surface, such as the surface of a windshield of a vehicle for an automobile, in order to automatically actuate the vehicle's windshield wipers and/or defroster or defogging system.
In conventional windshield wiper systems, the windshield wipers are actuated based on the elapsed time between wipes rather than the moisture level on the exterior of the windshield. During conditions of relatively consistent rainfall, for example, the time interval can be adjusted to correspond to the amount of time in which the rain accumulates to the point of the desired visibility level. Unfortunately, the rate of rainfall may vary dramatically over a given period of time. Additionally, traffic conditions may also cause varying amounts of rain to fall on the windshields, such as when a truck passes by. As a result, during such conditions, the driver must frequently adjust the wiper time interval, which can be cumbersome.
Various systems are known which automatically control the interval between wipes of the windshield wipers based upon moisture on the vehicle windshield. In some known systems, various coatings are applied to the vehicle windshield. Electrical measurement of those coatings is used to provide an indication of the moisture content on the windshield. Unfortunately, such methods require relatively expensive processes, which makes such systems commercially non-viable. Other systems for automatically sensing the moisture content on a vehicle windshield are also known. For example, optical systems are known which measure the difference of reflected light of a dry windshield versus a wet windshield. Unfortunately, such optical systems are susceptible to interference from external light sources and thus provide inadequate performance. Other known systems must be adhered to the windshield, which complicates the windshield replacement. As a result of such complications, moisture sensors are rarely found on vehicles.
Another system for automatically detecting the moisture content on a windshield is disclosed in Japanese Laid Open Patent Application No. Hei(1995)-286130, which describes the use of a charge coupled device (CCD) image sensor to image a portion of the vehicle windshield in order to detect raindrops. The system described therein computes the sum of the differences between each pixel and the average of all pixels. Unfortunately, headlamps of oncoming vehicles will create a bright spot in the image, which would be difficult to completely blur and likely be interpreted as rain. Moreover, in order for such a system to work effectively, distant objects within the imaged scene must be completely blurred. Otherwise, there will be dark and light regions in the imaged scene corresponding to the distant objects. Although there is no optical system disclosed in the Japanese laid open patent application for accomplishing this objective, it would be very difficult to develop an optical system to completely blur an oncoming headlamp. Failure to blur oncoming headlamps could cause false triggering of the system disclosed in the above-identified Japanese laid open patent application.
In commonly assigned U.S. Pat. Nos. 5,923,027 and 6,097,024, a moisture sensing system is disclosed that overcomes the problems noted above. Nevertheless, it would be desirable to further improve the ability of the disclosed system to distinguish from light sources within the imaged scene that may cause false triggering of the windshield wipers.
German Patent No. DE 198 03 694 C1 discloses a moisture sensing system in which two one-dimensional images are acquired of substantially the same portion of a vehicle windshield. The images are subtracted from one another to remove far field objects. In one embodiment, a single one-dimensional array of sensors is used and light from two different and offset light sources is sequentially directed towards the imaged portion of the windshield. The sensor array senses the reflections from any moisture on the windshield. A first image is acquired using the first light source and then stored, and subsequently, a second image is acquired using the second light source. The two images are then subtracted from one another to remove far field objects. This embodiment is highly dependent upon the ability of the system to distinguish the light from the two light sources from high levels of ambient light. Also, because the two images are acquired sequentially, there is a greater possibility that the far field objects in the two images will have moved and thus appear as a near field image.
German Patent No. DE 198 03 694 C1 discloses a second embodiment in which two separate, horizontally offset, one-dimensional image sensor arrays are used in combination with two separate lenses, to image the same portion of the windshield. While this embodiment overcomes some of the problems noted above with respect to the first embodiment, it increases the cost of the system by requiring two separate image sensor arrays. Neither of the two embodiments provides a very robust image from which useful information such as the spatial distribution of objects on the windshield may be obtained, or from which abnormalities in the detected light levels may be readily discarded.